by Pon » Thu Feb 06, 2020 4:12 pm
Hi, I'm the person who composed the image and the text. There seems to be a few questions regarding the post, allow me to try to explain some of them.
- All these images were taken with different exposure time, so it was impossible to perfectly line up all the shots. However, as the impact only lasts a fraction of a second (as shown by videos of the event), it is guarantee that these images do overlap at least during the time of the flash (or parts of it). Especially since there is only one flash recorded on the eclipsing Moon in the entire 2019, and only one other impact event during the lunar eclipse has been recorded so far.
Knowledge about the meteor impact:
- Impact of this size is impossible to predict. Therefore nobody knew about it before. As it turns out, meteor impacts DURING a total lunar eclipse is quite rare. All the photographers who captured it did not know about it, and most likely they were not even aware of it happening during capture. It was only after the impact became viral on the internet that they started looking to see if they had captured it. Luckily enough, since it was a rather rare and fortuitous event, many tried to submit it to APOD, which is how we were able to gather so many of these rare images.
- "Up" in space is rather tricky. For most of us Earthlings we refer to whichever direction that is perpendicular to the horizon as "Up". Therefore the notion of "Up" will depend on latitudes. However, for telescopes that do not see the horizon this notion means very little. Since "Up" will vary a lot depending on the latitude and time, I opted to go for a more universal "North" in orienting this picture.
Background Stars & Composite:
- To create this composite, first I calculate the angular size of each pixel for each image, as each images were taken by different focal length and different sensor. Then I apply scaling to each image so that it's all on the same angular scale. Then it was a matter of matching background stars so they all lined up. The background stars were indeed the ones that came with the individual photos (in fact, stars in each images were combined with lighten mode on photoshop). If you look at the lower left of the image you would not be seeing any stars there, simply because no photo overlap in that region. I just filled it with a blank space. As someone else already mentioned, those "BD" are star names.
- Libration is there, but it's small and very subtle. One way you can think of Libration is that the center part of the Moon is 1,737 km closer than the edge so it has slightly less parallax compared to the edge (in fact, it has about 4% smaller parallax angle). However, as it turns out, human eyes are really good at detecting parallax as that's what we do all the time. If you use cross-eye 3D technique on any two sufficiently far away pair you can actually see a spherical moon popping out with your own eyes! However, this technique might be tricky to do for most people. Alternatively, you could also put two images side by side on a VR (or a smartphone with the VR headset) and see it yourself as well.
Last edited by Pon on Thu Feb 06, 2020 4:18 pm, edited 1 time in total.